1. Field of the Invention
The present invention relates to a fuel cell system which generates electricity through the reaction between hydrogen and oxygen, and process for controlling the fuel cell in the fuel cell system. More particularly, the invention relates to a fuel cell system having improved performance during the transition period, at which the power generation amount is changed, and a process for producing the same.
2. Description of Related Arts
In recent years, an electric powered vehicles each carrying various driving motors instead of the conventional engine has been developed. One example of such types of electric powered vehicles includes a fuel cell carried vehicle having a Proton Exchange Membrane Fuel Cell abbreviated as “PEM FC” (hereinafter PEM type fuel cell or simply referred to as fuel cell) as a power source for a driving motor, and such PEM type fuel cell carried vehicles have been sharply developed.
PEM FC comprises a stack structure having a lot of single cells, which are power generation units, laminated on each other. Each cell has a configuration composed of an anode side separator having a hydrogen passage, a cathode side separator having an oxygen passage, and a membrane-electrode assembly (hereinafter abbreviated as “MEA”) intervened between these separators. MEA is composed of a proton exchange membrane abbreviated as PEM, each surface of PEM with a catalyst layer and a gas diffusion layer laminated one after another (one surface having an anode side catalyst layer and a gas diffusion layer and the other having a cathode side catalyst layer and a gas diffusion layer).
In such PEM FC, when hydrogen gas flows through the hydrogen passage from the inlet side to the outlet side of the anode and when air (as an oxidant gas) flows through the oxygen passage from the inlet side to the outlet side of the cathode, the protons permeate through PEM of MEA in a wet state from the anode side of each cell, migrating to the cathode side. This causes each cell to generate electromotive force of approximately 1 V. In PEM FC having such a power generation mechanism, air and hydrogen are continuously supplied to continue the power generation. Consequently, an air intake system, which compresses air, for example, by a compressor is provided at the inlet side of cathode, and an air exhaust system, for example, having a backpressure control valve, is provided on the outlet side of cathode. In addition, a hydrogen gas supply system, which supplies hydrogen by an ejector, is provided on the inlet side of anode.
As described above, in the fuel cell system having the air supply system, the air exhaust system, and the hydrogen supply system provided on the fuel cell, the revolution speed of the compressor is controlled to be increased or decreased by increasing or decreasing an amount of the air flowing to the cathode inlet, whereby a power generation amount (output current or output power) is controlled (increased or decreased). At this time, if the pressure difference between the poles, i.e., the difference between the hydrogen pressure and the air pressure, becomes unduly large, there is a fear of breaking PEM making up MEA. Consequently, the hydrogen gas pressure at the anode inlet side and the air pressure at the cathode inlet side are separately controlled so that the pressure difference between the poles falls within a tolerance range. Specifically, in the conventional fuel cell system, the revolution speed of the compressor is controlled to be a target value where the air-flow amount to the cathode inlet side is controlled to be a target air flow amount, and the opening of the backpressure control valve of the air exhaust system is controlled so that the air pressure becomes a target pressure.
Meanwhile, it takes a very short period that the opening of the backpressure control valve reaches a target value in comparison with the period that the air pressure reaches a target air pressure. However, in the conventional fuel cell system, the opening of the backpressure control valve is sharply controlled so as to be a target value corresponding to the target airflow amount. For example, as shown in FIG. 5, when the airflow amount Q is increased to a given target airflow amount QT, the opening γ of the backpressure control valve is sharply controlled to be a target value corresponding to the target airflow amount QT as shown in the broken line. For this reason, at the transition period until the airflow amount Q reaches the target airflow amount QT, the backpressure control valve is excessively wide-opened to the target opening corresponding to the target airflow amount QT in advance and, thus, the pressure P of the air to be compressively transferred toward the cathode inlet by a supercharger is escaped toward downstream of the backpressure control valve. As a result, the air pressure P at the cathode inlet side is once decreased and then reaches a target air pressure PT, conducting that the pressure increase is delayed. The behavior at the time where the airflow amount Q is decreased to a given target airflow amount QT is that the air pressure P is once increased, and then reaches the target air pressure PT during the transition period, delaying the decreasing of the pressure.
In the conventional fuel cell system as described above, during the transition period when the airflow amount at the cathode inlet side is increased or decreased to a target airflow amount corresponding to the decreasing or increasing of the power generation amount, the air pressure at the cathode inlet side is once decreased or increased. Accordingly, there poses a problem that pressure difference between the poles in the fuel cell system (pressure difference between the anode side and the cathode side applied to PEM of MEA) is increased. Also, the conventional fuel cell system is disadvantageous in that there is a time lag until the air pressure at the cathode inlet reaches a target air pressure, leading to poor responsibility.